1. Field of the Invention
The invention is directed to apparatus and methods for contacting gas and solids in the manufacture of edibles, and may be used for drying, drying combined with coating, heating, cooling, and cooling combined with coating.
2. Description of the Related Art
Application of a sugar layer to the exterior of a solid particle is a common practice in the confectionery and pharmaceutical arts. The commonly practiced batch panning process, in which sugar syrup is added to the solids shot by shot, is highly capital and labor-intensive, and a significant amount of sugar is carried away in the course of drying. There would be many practical advantages if such coating processes could be practiced on a continuous basis. However, apparatus for conducting this type of coating on a continuous basis have not gained widespread acceptance because they have not yielded acceptable product quality.
U.S. Pat. Nos. 6,364,948 and 6,689,417, which are owned by the assignee herein, are directed to methods and apparatus for batch coating and/or drying (including methods for making sugar shell coated products) in which particles are placed in a bed and displaced in the x, y and z directions by vibrating the bed so that the particles follow a dominant path or direction. Drying air is provided tangentially to the bed.
U.S. Pat. No. 6,638,550, owned by the assignee herein, is directed to methods for forming a shell coating on a mass of confectionery centers. The process may utilize a rotatable drum in which a current of drying gas is provided having a controlled moisture content.
U.S. Pat. No. 6,960,361, owned by the assignee herein, is directed to a panning apparatus for coating a plurality of center materials. The panning drum incorporates a partition, dividing the drum into a plurality of coating vessels.
Published International Application WO 99/26484 teaches an apparatus for batch coating chewing gum or other edible materials. The apparatus comprises a rotating drum having angled baffles on the inside. Liquid coating material is provided through an internal pipe running along the axis of the drum, and drying air is provided through a plurality of air distribution chambers arranged around the periphery of the drum. Air supply and removal conduits are provided so that drying air travels axially in the drum.
U.S. Pat. No. 6,365,203 B2 and No. 7,022,353 B2 teach a method and apparatus for continuous coating of gum materials. The coating material may be a hardenable sugar solution. The apparatus comprises a rotating drum which is angled with respect to the horizontal so that the edible pieces move through the drum by the force of gravity. The residence time is shortened or increased by raising or lowering the angle of the drum. Liquid coating material is fed to the drum through a central conduit along the principle axis of the drum, while heated drying air is preferably introduced through perforations in the side walls of the drum, to provide a uniform flow of drying air in the drum.
U.S. Pat. No. 5,520,457 and EP 0 192 012 teach coating apparatus and methods for coating particulate solids in which a rotating spiral brush is used to push material through a drum, which may also rotate. These apparatus and methods do not contemplate a system wherein a source of gas is provided to a substantially closed gas pathway defined by a spiral element.
WO 2005/011398 discloses a method for continuously coating cores in a dragee making process, which comprises transporting the cores through a rotating drum with a screw-like conveyor.
None of the foregoing disclosures effectively makes use of a closed gas path for contacting solids with gas in a controlled, continuous manner. A significant problem with prior art methods and apparatus for coating particulate solids concerns the handling of dust formed by the tumbling action of the solids. The dust becomes airborne and may damage humidity control systems. Expensive dust filtration equipment may be required, increasing both the complexity of the process and maintenance costs. This problem may be solved according to the present invention by directing the flow of process gas to ensure that dust formed by tumbling the solids is adhered to the solids before the solids are discharged from the apparatus.